Composite laminate and load-introduction component for a load-introduction joint

ABSTRACT

A composite laminate for connection to at least one attachment component in a load-introduction joint, said composite laminate comprising an opening that is adapted to receive a load-introduction component of said load-introduction joint, wherein a prefailure indication element is provided in the region of said opening, said prefailure indication element being modifiable by said load-introduction component in a prefailure mode of said load-introduction joint for indication of an impending function-affecting failure of said composite laminate. The invention is further related to a corresponding load-introduction component.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 14/877,077 filed Oct. 7, 2015, which, in turn, claims priority to European Patent Application No. 14 400047.8 filed on Oct. 8, 2014, the disclosures of which are incorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The invention is related to a composite laminate for connection to at least one attachment component in a load-introduction joint by means of a load-introduction component, said composite laminate comprising the features of the claims. The invention is further related to a load-introduction component for attachment of at least one attachment component to a composite laminate in a load-introduction joint, said load-introduction component comprising the features of the claims.

(2) Description of Related Art

A load-introduction joint generally defines a connection between at least two different components, wherein a load is introduced from at least one first component into at least one second component. For instance, in aerospace engineering such a load-introduction joint is amongst others embodied between a rotor blade and an associated rotor head of a rotor in a rotary-wing aircraft, wherein the rotor blade is joined to the rotor head by at least one suitable load-introduction component.

More specifically, in commonly used rotary-wing aircrafts the rotor blades are usually embodied as composite laminates made from composite materials, such as fiber reinforced polymers (FRP), which are joined to a respective rotor head by means of bolts. In operation of such a rotary-wing aircraft, loads acting on a given rotor blade are, thus, applied to the composite laminate defining the rotor blade and act on the composite laminate in essentially one main load direction that is defined by the longitudinal axis of the rotor blade. These loads are introduced from the composite laminate in the main load direction into the rotor head by means of at least one bolt, which defines the load-introduction component in the corresponding load-introduction joint.

The above described load-introduction joint is implemented as a bolted connection, as the load introduction into the composite laminate is performed by means of at least one bolt acting as load-introduction component. In such a bolted connection with a single main load direction, an underlying diameter d of the bolt, a diameter D of an opening provided in the composite laminate for reception of the bolt, a thickness t of the composite laminate, a distance e1 from the center of the opening to an edge of the composite laminate that is arranged in parallel to the main load direction, and a distance e2 from the center of the opening to an adjacent edge of the composite laminate that is arranged perpendicular to the main load direction, are considered as critical dimensions in the design of the bolted connection.

However, it should be noted that in cases, where the bolted connection comprises multiple bolts that are arranged in close relationship with respect to each other, only half of a given distance between the centers of two neighboring openings provided in the composite laminate are used for the determination of e1 or e2, depending on underlying relative positions of the neighboring openings on the composite laminate and the main load direction. Furthermore, in cases without a single main load direction, a smallest determinable edge distance e_min is considered.

The above described critical dimensions are considered during manufacturing of bolted connections, i.e. constituent components thereof. Conventional bolted connections with a composite laminate are usually realized according to one of two different main manufacturing techniques. These two different main manufacturing techniques are briefly described hereinafter.

The document JP 2012/061672 A describes a bolted connection that is embodied according to a first manufacturing technique with a composite laminate having fibers that are arranged in a loop around an opening in the composite laminate. The fibers are implemented as straight lines towards or away from the opening from or into the composite laminate. The opening is adapted to receive a load-introducing bolt, such that an elasto-mechanical mechanism is created which transforms radial forces resulting from a contact between the opening and the bolt into tangential forces in the composite laminate around the opening. Stresses that are directly resulting from these radial forces are referred to as “bearing stresses”, while stresses that are resulting from the tangential forces are referred to as “tangential stresses”.

In such a bolted connection that is embodied according to this first manufacturing technique, the orientations of the tangential stresses and the fibers are essentially parallel to each other, as the fibers are arranged in a loop around the opening. Thus, the composite laminate has its highest strength in parallel to the fiber orientations. Furthermore, in adequately designed loops the tangential stresses are larger than the bearing stresses and the elasto-mechanical mechanism that transforms the radial forces into the tangential forces results in shear stresses, which act between the fibers.

However, manufacturing of such a composite laminate having fibers that are arranged in a loop around an opening in the composite laminate is generally complicated, time-consuming and expensive, as it is difficult, and often impossible, to automate fiber placement during manufacturing, so that usually manual fiber placement is required. Furthermore, an available design space for such composite laminates is constrained, as usually comparatively small relative edge distances with e/D<2 are requested, so that a proper design of a composite laminate that is suitable to bear high loads would require large thicknesses t of the composite laminate.

The document US 2012/0045613 A1 describes a bolted connection that is embodied according to a second manufacturing technique with a composite laminate having multiple layers. Each layer comprises straight fiber orientations or is embodied as a fabric with two straight fiber orientations. More specifically, the fiber orientations in the multiple layers of the composite laminate are selected in order to bear the different load components described above, i.e. bearing stresses, tangential stresses and shear stresses. Accordingly, the fibers can be unidirectional, woven, knitted, braided, stitched, and so on.

In such a bolted connection that is embodied according to the second manufacturing technique, the composite laminate is provided with an opening that is adapted to receive a load-introducing bolt. The composite laminate as such is generally referred to as “bearing laminate”.

However, such a bearing laminate has associated failure modes that may occur abruptly in operation and, thus, affect safety of use of the bearing laminate. In order to increase the safety of use, an implementation of gradual failure modes is required. This can, e.g., be achieved by designing the bearing laminate with comparatively large relative edge distances of e/D>2˜3, depending on the type of bearing laminate, and small thicknesses t of the bearing laminate.

It should be noted that alternative manufacturing techniques of composite laminates for bolted connections are also known. Such alternative manufacturing techniques are, by way of example, described in the documents DE 10 2006 001 444 A1, DE 10 2006 007 428 A1 and DE 10 2010 009 769 A1. Furthermore, the above described first and second manufacturing techniques can be combined, so that composite laminates consisting of layers that are embodied as loops and layers that are embodied as bearing laminates are obtained.

However, all such manufacturing techniques are complicated, time-consuming and expensive, or result in composite laminates with comparatively large relative edge distances.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a composite laminate for a bolted connection and, more generally, for a load-introduction joint, said composite laminate being comparatively easy and inexpensive to manufacture and adapted to increase safety in operation. This object is solved by a composite laminate for connection to at least one attachment component in a load-introduction joint, said composite laminate having the features of claim 1.

More specifically, according to the present invention a composite laminate for connection to at least one attachment component in a load-introduction joint comprises an opening that is adapted to receive a load-introduction component of said load-introduction joint. A prefailure indication element is provided in the region of said opening. Said prefailure indication element is modifiable by said load-introduction component in a prefailure mode of said load-introduction joint for indication of an impending function-affecting failure of said composite laminate. Preferably, said prefailure indication element is modifiable by said load-introduction component in said prefailure mode of said load-introduction joint such that said prefailure indication element is at least compressible and/or at least partly destructible by said load-introduction component.

Advantageously, by providing the prefailure indication element, an impending function-affecting failure of the composite laminate can be detected safely and reliably prior to its occurrence and may, therefore, be avoided. In other words, by providing the prefailure indication element, extra time for detection of an impending function-affecting failure of the composite laminate is gained. Thus, safety and reliability of load-introduction joints with composite laminates can be increased substantially.

According to one aspect, a given bearing surface between the composite laminate and the load-introduction component in the load-introduction joint is reduced by modifying a cylindrical interface between the load-introduction component, such as a bolt or a bushing, and the opening in the composite laminate. This is preferably achieved by creating one or more cut-outs on at least one side of the opening in the composite laminate for reducing a given contact surface between the bolt or bushing and the composite laminate in the load-introduction joint, which is subject to normal loads in operation due to a respectively applied external load. The cut-outs are created such that, e.g., at least one prefailure indication projection is formed that defines the inventive prefailure indication element.

Preferably, said at least one prefailure indication projection comprises a projection thickness that amounts at least to 50% and at most to 90% of a thickness of said composite laminate. More specifically, said at least one prefailure indication projection preferably comprises a length that amounts to at least 0.5% of a given diameter D of the opening, if D>40 mm, and that amounts maximally to 50% of said given diameter D, if D<4 mm, said length being preferentially comprised in a range from 0.2 mm to 2 mm. Furthermore, said at least one prefailure indication projection preferably comprises a constituting material that is weaker than a constituting material of said composite laminate.

However, it should be noted that this inventive prefailure indication element is not necessarily formed by one or more cut-outs in the form of said prefailure indication projection. For instance, at least two prefailure indication projections can be provided that are defined by peripheral recesses, instead of cut-outs, that are embodied in the region of said opening. Other embodiments are also possible and contemplated within the context of the present invention. For instance, one or more of the layers defining the composite laminate, which have a comparatively high stiffness normal to the cylindrical surface of the opening and which have a comparatively high bearing strength, can locally be replaced with prefailure layers having a low stiffness and a low strength normal to said cylindrical surface. This increases the bearing stress in the remaining high stiffness layers, which contribute most to the overall bearing strength.

According to one aspect of the present invention, at least one prefailure layer is mounted into the opening of the composite laminate. Said at least one prefailure layer preferably comprises a laminate section with dummy layers that are arranged in an underlying prefailure zone.

Advantageously, compared to composite laminates that are embodied according to the above described loop design, the inventive composite laminate can be produced using automated processes. This leads to a cheaper manufacturing, but with comparable strengths, and also allows for an easier manufacturing, e.g. using resin transfer processes.

Furthermore, by providing the inventive composite laminate with the prefailure indication element that is adapted for indicating an impending function-affecting failure of said composite laminate in an associated prefailure mode, the available design space for efficient and safe application of composite laminates in load-introduction joints can be expanded. In particular, it allows composite laminate designs with smaller relative edge distances e/D compared to e.g. conventional bearing laminates as described above. Such smaller relative edge distances e/D advantageously result in smaller stress concentration factors for the shear stresses and tangential stresses that may occur in the composite laminate in operation. These smaller relative edge distances allow manufacturing of composite laminates that are, at a same load level for function-affecting failure, more light weight than conventional bearing laminates that are designed with detectable prefailure modes.

In operation of a load-introduction joint with a composite laminate according to the present invention, the full cylindrical surface of the opening in the region of the prefailure indication element only comes into contact with the bolt or bushing after the prefailure indication element, i.e. the material defining the prefailure indication element, has failed, i.e. in the prefailure mode. In other words, in a normal operation mode of the load-introduction joint, the bearing strength of the composite laminate is reduced and only in the prefailure mode, i.e. after failure of the prefailure indication element has occurred, the full bearing strength is obtained. Thus, further progression of the failure is at least temporarily stopped or slowed down, which expands a detection time interval, in which the prefailure and, thus, an impending function-affecting failure of said composite laminate can be detected and accordingly provides the extra time as described above.

According to a preferred embodiment, said prefailure indication element is provided in the region of said opening at least on a peripheral side of said opening that is arranged diametrically opposed to a main load direction of a main load that is applicable to said composite laminate in operation.

According to a further preferred embodiment, said prefailure indication element comprises at least one prefailure indication projection that is arranged in said opening.

According to a further preferred embodiment, said at least one prefailure indication projection is arc-shaped.

According to a further preferred embodiment, said at least one arc-shaped prefailure indication projection is arranged between a first and a second arc-shaped axial recess provided in the region of said opening.

According to a further preferred embodiment, said at least one arc-shaped prefailure indication projection is arranged between a first and a second tangential recess provided in the region of said opening.

According to a further preferred embodiment, said at least one prefailure indication projection is annular.

According to a further preferred embodiment, said at least one annular prefailure indication projection is arranged between a first and a second axial chamfer provided on each axial end of said opening.

According to a further preferred embodiment, said at least one prefailure indication projection comprises at least one undercut that is adapted to divide said at least one prefailure indication projection into first and second projection sections.

The present invention further provides a load-introduction component for attachment of at least one attachment component to a composite laminate in a load-introduction joint. Said load-introduction component comprises a cylindrical section that is adapted to be received in an opening provided in said composite laminate. A prefailure indication element is provided on said cylindrical section. Said prefailure indication element is adapted to modify said composite laminate in a prefailure mode of said load-introduction joint for indication of an impending function-affecting failure of said composite laminate.

According to one aspect, a given bearing surface between the load-introduction component and the composite laminate in the load-introduction joint is reduced by modifying a cylindrical interface between the load-introduction component and the opening in the composite laminate. This is preferably achieved by creating one or more cut-outs and/or recesses on at least one peripheral side of the load-introduction component for reducing a given contact surface between the load-introduction component and the composite laminate in the load-introduction joint, which is subject to normal loads in operation of the load-introduction joint due to a respectively applied external load.

According to a preferred embodiment, said prefailure indication element comprises at least one annular recess provided on said cylindrical section.

According to a further preferred embodiment, said prefailure indication element comprises at least one peripheral cut-out provided on said cylindrical section. Preferably, two peripheral cut-outs are provided, said two peripheral cut-outs being arranged in mirror-symmetry relative to each other.

According to a further preferred embodiment, said at least one peripheral cut-out is arranged in parallel to a middle axis of said load-introduction component.

According to a further preferred embodiment, said load-introduction component is embodied as a bolt, wherein said cylindrical section is embodied as a bolt shaft.

According to a further preferred embodiment, said load-introduction component is embodied as a bushing, wherein said cylindrical section is embodied as a sleeve-like area. Preferably, said bushing comprises an outer circumference that is arranged concentrically or eccentrically to an inner opening defined by said sleeve-like area. According to one aspect, said bushing comprises an elliptical outer circumference.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Preferred embodiments of the invention are outlined by way of example in the following description with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labeled with identical reference numbers and characters and are, consequently, only described once in the following description.

FIG. 1 shows a sectional view of a load-introduction joint with a composite laminate and a load-introduction component according to a first embodiment of the invention,

FIG. 2 shows a top view of the composite laminate of FIG. 1, seen in direction of an arrow II of FIG. 1,

FIG. 3 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2,

FIG. 4 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows IV-IV of FIG. 2,

FIG. 5 shows a sectional view of the composite laminate of FIG. 2 and FIG. 3 in a normal operation mode and in the prefailure mode, seen in direction of arrows V-V of FIG. 3,

FIG. 6 shows the composite laminate and the load-introduction component of FIG. 1 in a first possible function-affecting failure mode,

FIG. 7 shows the composite laminate and the load-introduction component of FIG. 1 in a second possible function-affecting failure mode,

FIG. 8 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a second embodiment,

FIG. 9 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a third embodiment,

FIG. 10 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a fourth embodiment,

FIG. 11 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a fifth embodiment,

FIG. 12 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a sixth embodiment,

FIG. 13 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a seventh embodiment,

FIG. 14 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to an eighth embodiment,

FIG. 15 shows a simplified sectional view of the load-introduction joint of FIG. 1, with a load-introduction component according to a second embodiment,

FIG. 16 shows a sectional view of the load-introduction component of FIG. 1 according to a third embodiment,

FIG. 17 shows a sectional view of the load-introduction component of FIG. 1 according to a fourth embodiment,

FIG. 18 shows a sectional view of the load-introduction component of FIG. 1 according to a fifth embodiment,

FIG. 19 shows a sectional view of the load-introduction component of FIG. 1 according to a sixth embodiment, and

FIG. 20 shows a sectional view of the load-introduction component of FIG. 1 according to a seventh embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a load-introduction joint 1 according to the present invention, which comprises a composite laminate 6 that is connected to at least one, and illustratively two attachment components 4, 5 by means of at least one associated load-introduction component. The latter is adapted to introduce loads that are applied to the composite laminate 6 into the attachment components 4, 5.

By way of example, the load-introduction joint 1 can be used to connect a rotor blade of a multi-blade rotor of a rotary-wing aircraft to a rotor head of said multi-blade rotor. In this case, the composite laminate 6 is associated with, or integrated into, the rotor blade and the attachment components 4, 5 are associated with the rotor head and, e.g., integrated into a flexbeam thereof.

According to one aspect, the load-introduction component comprises a bolt 2 with a bolt shaft 2 a and three bushings 8, 9, 23. A first bushing 8 is arranged in an associated opening 4 a provided in the attachment component 4, a second bushing 9 is arranged in an associated opening 5 a provided in the attachment component 5, and a third bushing 23 is arranged in an associated opening 6 a provided in the composite laminate 6. All three bushings 8, 9, 23 are equipped with corresponding openings 8 a, 9 a, 23 a, respectively, wherein the bolt shaft 2 a with an outer circumference 2 b is arranged. The bolt 2 is secured in the bushings 8, 9, 23 by means of a nut 3 and, thus, secures the bushings 8, 9, 23 to the attachment components 4, 5 and the composite laminate 6 on the one hand and, on the other hand, attaches the composite laminate 6 to the attachment components 4, 5.

In operation of the load-introduction joint 1, a main load F that is, e.g., applicable to the composite laminate 6 in a main load direction 10, which illustratively corresponds to a longitudinal extension of the composite laminate 6, is introduced from the composite laminate 6 via the bushing 23 into the bolt 2, i.e. the bolt shaft 2 a, and from the bolt shaft 2 a via the bushings 8, 9 into the attachment components 4, 5, respectively. Thus, in a strict sense the third bushing 23 defines the load-introduction component in the load-introduction joint 1 with respect to the composite laminate 6.

However, it should be noted that use of the three bushings 8, 9, 23 is only optional and not mandatory for realization of the present invention. In fact, the three bushings 8, 9, 23 can alternatively be implemented as only one or two bushings or, still alternatively, be omitted, if the underlying dimensions of the openings 4 a, 5 a, 6 a provided in the attachment components 4, 5 and the composite laminate 6 match the underlying dimensions of the bolt shaft 2 a, i.e. its outer circumference 2 b, at least within usual predetermined manufacturing tolerances that allow to press-fit or screw the bolt 2 into the attachment components 4, 5 and the composite laminate 6. In this case, the bolt 2 defines the load-introduction component of the load-introduction joint 1. Therefore, the load-introduction component of the load-introduction joint 1 is hereinafter referred to with the reference sign “2”, for simplicity.

Illustratively, the composite laminate 6 with the opening 6 a comprises an upper side 6 b and an opposed lower side 6 c and is preferably embodied similar to a bearing laminate. On the upper side 6 b, the attachment component 4 with the opening 4 a is arranged. Therefore, the attachment component 4 is hereinafter referred to as the “upper” component, for simplicity and brevity. On the lower side 6 c, the attachment component 5 with the opening 5 a is arranged. Therefore, the attachment component 5 is hereinafter referred to as the “lower” component, for simplicity and brevity.

The opening 4 a of the upper component 4 has dimensions that preferably match corresponding dimensions of an outer circumference 8 b of a sleeve-like area 8 c of the first bushing 8, at least within usual predetermined manufacturing tolerances in analogy to what is said above. It should be noted that the sleeve-like area 8 c illustratively merges into a flange-like extension arranged essentially between the upper component 4 and the nut 3. However, this flange-like extension is only optional and, therefore, not specifically referenced, for simplicity and clarity of the drawings.

The opening 5 a of the lower component 5 has dimensions that preferably match corresponding dimensions of an outer circumference 9 b of a sleeve-like area 9 c of the second bushing 9, at least within usual predetermined manufacturing tolerances in analogy to what is said above. It should be noted that the sleeve-like area 9 c illustratively merges into a flange-like extension arranged essentially between the lower component 5 and a bolt head 2 d of the bolt 2. However, this flange-like extension is only optional and, therefore, not specifically referenced, for simplicity and clarity of the drawings.

According to one aspect of the present invention, the opening 6 a of the composite laminate 6 is provided with a prefailure indication element 7. More specifically, this prefailure indication element 7 is preferably provided in the region of the opening 6 a, preferentially at least on a mainly loaded side thereof, and modifiable by the load-introduction component 2 in a prefailure mode of the load-introduction joint 1 for indication of an impending function-affecting failure of the composite laminate 6. Such a modification of the prefailure indication element 7 by the load-introduction component 2 in the prefailure mode comprises, for instance, an at least partial compression and/or destruction of the prefailure indication component 7.

In other words, the prefailure indication element 7 is adapted to fail without affecting the overall function of the load-introduction joint 1. However, after such a failure of the prefailure indication element 7, a subsequent function-affecting failure of the load-introduction joint 1 as such must reasonably be expected. Therefore, the load-introduction joint 1 is considered being in a “prefailure mode” after failure of the prefailure indication element 7.

Preferably, the prefailure indication element 7 is provided at least on a peripheral side 6 g of the opening 6 a that is arranged diametrically opposed to the main load direction 10 of the main load F and, thus, defines the mainly loaded side of the opening 6 a. The prefailure indication element 7 can be realized by one or more prefailure indication projections and is, illustratively, realized by a single prefailure indication projection 7 a that is arranged in the opening 6 a. In the region of the single prefailure indication projection 7 a, the opening 6 a has dimensions that preferably match corresponding dimensions of an outer circumference 23 b of a sleeve-like area 23 c of the third bushing 23, at least within usual predetermined manufacturing tolerances in analogy to what is said above.

According to a first embodiment, the single prefailure indication projection 7 a is arc-shaped and provided only on the mainly loaded side of the opening 6 a, as illustrated in greater detail in FIG. 2. This arc-shaped prefailure indication projection 7 a is preferably arranged between a first and a second arc-shaped axial recess 7 b, 7 c provided in the region of the opening 6 a.

FIG. 2 shows the upper side 6 b of the composite laminate 6 of FIG. 1, which is provided with the opening 6 a and the prefailure indication element 7 with the prefailure indication projection 7 a. The latter illustratively comprises a maximal length 11 e in the main load direction 10 of FIG. 1.

The opening 6 a has a center 6 d and an opening diameter 11 c. The center 6 d is illustratively arranged on a longitudinal axis of the composite laminate 6, which is coaxial to the main load direction 10 of FIG. 1, by way of example.

The composite laminate 6 comprises longitudinal edges between its upper side 6 b and its lower side 6 c of FIG. 1, which are illustratively oriented in parallel to the main load direction 10 and from which only the—in FIG. 2—upper longitudinal edge is labelled with the reference sign 6 f. The composite laminate 6 further comprises end edges between its upper side 6 b and its lower side 6 c of FIG. 1, which are illustratively oriented perpendicular to the main load direction 10 and from which only the—in FIG. 2—left end edge is shown and labelled with the reference sign 6 e.

Between the end edge 6 e and the opening center 6 d, an end edge distance 11 a is embodied. Between the longitudinal edge 6 f and the opening center 6 d, a longitudinal edge distance 11 b is embodied. The relation between the end edge distance 11 a or the longitudinal edge distance 11 b and the opening diameter 11 c is preferably less than 2 and, illustratively, equal to 1.

Furthermore, according to one aspect of the present invention, the length 11 e of the prefailure indication projection 7 a is determined dependent on the opening diameter 11 c. More specifically, the length 11 e preferably amounts to at least 0.5% of the opening diameter 11 c, if the latter is greater than 40 mm. If, however, the opening diameter 11 c is less than 4 mm, the length 11 e preferably amounts maximally to 50% of said opening diameter 11 c. More generally, the length 11 e is preferentially comprised in a range from 0.2 mm to 2 mm.

FIG. 3 shows the composite laminate 6 with the opening 6 a and the prefailure indication element 7 with the prefailure indication projection 7 a of FIG. 1 and FIG. 2 in greater detail. FIG. 3 further illustrates the axial recesses 7 b, 7 c of FIG. 1 that delimit the prefailure indication projection 7 a.

Illustratively, the composite laminate 6 exhibits between its upper and lower sides 6 b, 6 c a thickness 11 d and the prefailure indication projection 7 a exhibits a projection thickness 7 d. Preferably, this projection thickness 7 d comprises 50% to 90% of the thickness 11 d of the composite laminate 6, and illustratively amounts to 50% thereof.

FIG. 4 shows the composite laminate 6 with the opening 6 a of FIG. 1 to FIG. 3 to further illustrate the opening diameter 11 c and the projection thickness 7 d of the prefailure indication projection 7 a according to FIG. 3.

FIG. 5 shows the composite laminate 6 of FIG. 1 to FIG. 4 in a normal operation mode and in the prefailure mode according to the present invention. In the normal operation mode, the bolt shaft 2 a of the bolt 2 of FIG. 1 and/or the sleeve-like area 23 c of the bushing 23 of FIG. 1 are arranged in an initial position 12 a in the opening 6 a of the composite laminate 6 and the prefailure indication element 7, i.e. the prefailure indication projection 7 a of FIG. 1 to FIG. 4, is intact, i.e. unmodified.

In contrast thereto, the prefailure indication projection 7 a is modified i.e. at least partly compressed and/or destroyed, in the prefailure mode, wherein the bolt shaft 2 a of the bolt 2 of FIG. 1 and/or the sleeve-like area 23 c of the bushing 23 of FIG. 1 are arranged in a prefailure indication position 12 b. In this prefailure indication position 12 b, a gap 13 is formed between the composite laminate 6 and the bolt 2 and/or the bushing 23 in the opening 6 a. This gap 13 is preferably adapted to allow for an easy and reliable detection of the prefailure mode.

It should be noted that the gap 13 as such is not directly detectable, as it is covered by the upper and lower components 4, 5 in FIG. 1, However, if the gap 13 is formed a relative positioning of the composite laminate 6 with respect to these upper and lower components 4, 5 is changed and easily detectable.

FIG. 6 and FIG. 7 show the composite laminate 6 of FIG. 5 in the prefailure mode. For simplicity, only the bolt shaft 2 a of the bolt 2 of FIG. 1 is respectively illustrated in the opening 6 a of the composite laminate 6 and arranged in the prefailure indication position 12 b according to FIG. 5.

As indicated above, the prefailure mode alerts and indicates an impending function-affecting failure of the composite laminate 6. Such a function-affecting failure can e.g. be caused by transversal ruptures 14 a occurring in the composite laminate 6 according to FIG. 6, and/or by longitudinal ruptures 14 b occurring in the composite laminate 6 according to FIG. 7. However, by detecting the prefailure mode by means of the gap 13, an impending occurrence of such transversal and/or longitudinal ruptures can be predicted and counter-measures can be taken in advance, e.g. by exchanging the composite laminate 6.

It should be noted that in the composite laminate 6 according to the first embodiment, which is described as such in detail above with reference to FIGS. 1 to 4, the prefailure indication element 7 and, more particularly, the prefailure indication projection 7 a is implemented by forming the axial recesses 7 b, 7 c inside the opening 6 a. These axial recesses 7 b, 7 c are illustratively C-shaped in cross-section and preferably produced during manufacture of the composite laminate 6, e.g. by adequately forming layers that are lying upon each other. Alternatively, the axial recesses 7 b, 7 c can be formed by producing elongated blind holes in the opening 6 a. Furthermore, only the single main load direction 10 was assumed. However, other shapes and production techniques and also more than one main load directions are also contemplated, as described below with reference to FIGS. 8 to 14.

FIG. 8 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to a second embodiment. In this second embodiment, the prefailure indication projection 7 a is arc-shaped and arranged between a first and a second axial chamfer 15 a, 15 b that are provided on each axial end of the opening 6 a. However, these axial chamfers 15 a, 15 b, and consequently also the arc-shaped prefailure indication projection 7 a, are only provided on a mainly loaded side of the opening 6 a, i.e. illustratively adjacent to the end edge 6 e.

FIG. 9 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a with the axial recesses 7 b, 7 c that are provided in the opening 6 a and now embodied according to a third embodiment. In this third embodiment, the axial recesses 7 b, 7 c, and consequently also the prefailure indication projection 7 a, are again arc-shaped and only provided on a mainly loaded side of the opening 6 a, i.e. illustratively adjacent to the end edge 6 e, like in the first embodiment. Alternatively, they can be annular in the case of multiple possible load directions.

However, the axial recesses 7 b, 7 c are smaller than in the first embodiment. Furthermore, the prefailure indication projection 7 a comprises at least one undercut 16 that is adapted to divide said at least one prefailure indication projection 7 a into first and second projection sections. Thus, a more even pressure distribution on the prefailure indication projection 7 a can be achieved in operation, or more pressure near the end edge 6 e of the composite laminate 6 is achievable so that a higher strength against bending moments can be realized.

In other words, by dividing the at least one prefailure indication projection 7 a into first and second projection sections, two prefailure indication projections are created, which are illustratively arranged in parallel to the longitudinal extension of the composite laminate 6. More generally, two or more prefailure indication projections can be defined by peripheral recesses and/or undercuts that are provided in the region of the opening 6 a, independent on whether the peripheral recesses and/or undercuts are oriented in parallel, perpendicular or transversely to the longitudinal extension of the composite laminate 6. Such a configuration can likewise be obtained by thinning out the composite laminate 6 in the region of the prefailure indication element 7.

FIG. 10 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to a fourth embodiment. In this fourth embodiment, the prefailure indication projection 7 a is arranged between a first and a second axial chamfer 15 a, 15 b that are provided on each axial end of the opening 6 a, like in the second embodiment. However, in contrast to this second embodiment, the axial chamfers 15 a, 15 b, and the prefailure indication projection 7 a are now annular. This is particularly useful if multiple load directions are possible instead of the single main load direction 10 of FIG. 1.

FIG. 11 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to a fifth embodiment. In this fifth embodiment, the prefailure indication projection 7 a is illustratively arc-shaped like in the first embodiment. However, in contrast to the first embodiment, the prefailure indication projection 7 a now comprises at least one prefailure layer 17 that is mounted into the opening 6 a. Preferably, the prefailure layer 17 comprises a laminate section with one or more dummy layers that define the prefailure indication projection 7 a.

More specifically, in the composite laminate 6 an underlying bearing force is mainly carried by layers with high stiffness and high strength in the normal direction of the bearing stress. The bearing stress in these layers can be increased by locally replacing some of the layers with dummy layers that have a low stiffness and low strength in the normal direction of the bearing stress, thereby defining the prefailure layer 17. This can, e.g., be reached by replacing some of the 0° layers, which are oriented parallel to the main load direction 10 of FIG. 1, with other layers, e.g., 90° layers, or non-FRP layers.

FIG. 12 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to a sixth embodiment. In this sixth embodiment, the prefailure indication projection 7 a is again arc-shaped and only provided on a mainly loaded side of the opening 6 a, i.e. illustratively adjacent to the end edge 6 e, like in the first embodiment. However, the prefailure indication projection 7 a is now arranged between a first and a second tangential recess 18 provided in the region of the opening 6 a on said mainly loaded side. Illustratively, the tangential recesses 18 are ear-shaped.

FIG. 13 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to a seventh embodiment. In this seventh embodiment, the prefailure indication projection 7 a is again annular and, thus, particularly suitable for applications with more than one main load direction.

However, in contrast to the sixth embodiment, multiple prefailure indication projections 7 a are now arranged between multiple tangential recesses 18 that are preferably arranged equidistantly around the opening 6 a. The number of tangential recesses 18 and corresponding prefailure indication projections 7 a is application-specific and amongst others dependent on an underlying number of possible load directions.

FIG. 14 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7 a that is provided in the opening 6 a and now embodied according to an eighth embodiment. In this eighth embodiment, the prefailure indication projection 7 a is illustratively arc-shaped like in the first embodiment. However, in contrast to the first embodiment, the prefailure indication projection 7 a now comprises a constituting material that is weaker than a constituting material of the composite laminate 6.

It should be noted that the prefailure indication element 7 is always described above with reference to FIGS. 1 to 14 as being realized by the composite laminate 6. However, other realizations are also possible and likewise contemplated, as described hereinafter with reference to FIGS. 15 to 20.

FIG. 15 shows the load-introduction joint 1 of FIG. 1 with the upper and lower components 4, 5, the composite laminate 6, the associated load-introduction component and the prefailure indication element 7. As described above, the load-introduction component is either embodied by the bolt 2 with the bolt shaft 2 a, or by the bushing 23, which are here exemplarily illustrated as a single component, for simplicity and clarity of the drawings.

According to one aspect of the present invention, the prefailure indication element 7 is provided on a cylindrical section of the load-introduction component, i.e. on an outer circumference 2 b of the bolt shaft 2 a of the bolt 2 or on an outer circumference 23 b of the bushing 23. Preferably, the prefailure indication element 7 comprises at least one and, preferentially, multiple annular recesses 19 provided on said cylindrical section. Thus, a contact surface between the bolt 2, or the bushing 23, and the composite laminate 6 in the opening 6 a can be reduced in analogy to what is said above.

It should be noted that such a reduction of the contact surface can be achieved by performing multiple different modifications to the bolt 2 or the bushing 23. Therefore, various modified bolts 2 and bushings 23 are shown in FIGS. 16 to 20 and described below, by way of example. However, it should be noted that corresponding modifications can likewise be applied to either one of the bolt 2 or the bushing 23, so that in each one of FIGS. 16 to 20 respectively only a single component is shown that simultaneously and schematically represents the bolt 2 and the bushing 23.

FIG. 16 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2 a, which is at least partly embodied with a varying or reduced outer circumference 2 c, e.g. by means of an annular recess. The reduced outer circumference 2 c and the normal outer circumference 2 b of the bolt shaft 2 a as such are preferably arranged concentrically.

On the other hand, FIG. 16 represents the bushing 23 of FIG. 15, which comprises the opening 23 a. The latter and the sleeve-like area 23 c of the bushing 23 are preferably arranged concentrically.

FIG. 17 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2 a, which is at least partly embodied with at least one peripheral cut-out 20 provided on its cylindrical section, i.e. on the outer circumference 2 b of the bolt shaft 2 a. On the other hand, FIG. 17 represents the bushing 23 of FIG. 15, which comprises said at least one peripheral cut-out 20 in its sleeve-like area 23 c.

In both cases, the at least one peripheral cut-out 20 is arranged in parallel to a middle axis 20 a of the bolt 2 or the bushing 23. Furthermore, by way of example two peripheral cut-outs 20 are provided and arranged in mirror-symmetry relative to each other.

FIG. 18 represents on the one hand the bolt 2 of FIG. 17 with the bolt shaft 2 a, which is at least partly embodied with multiple peripheral cut-outs 20 provided on its cylindrical section, i.e. on the outer circumference 2 b of the bolt shaft 2 a. On the other hand, FIG. 18 represents the bushing 23 of FIG. 17, which comprises said multiple peripheral cut-outs 20 in its sleeve-like area 23 c.

In both cases, the multiple peripheral cut-outs 20 are preferably arranged equidistantly around the middle axis 20 a of the bolt 2 or the bushing 23. The bolt 2 or bushing 23 according to FIG. 18 is particularly suitable for applications with more than one main load direction.

FIG. 19 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2 a, which is at least partly embodied with a varying or reduced outer circumference 2 c, e.g. by means of an annular recess. The reduced outer circumference 2 c and the normal outer circumference 2 b of the bolt shaft 2 a as such are preferably arranged eccentrically.

On the other hand, FIG. 19 represents the bushing 23 of FIG. 15, which comprises the opening 23 a. The latter and the sleeve-like area 23 c of the bushing 23 are preferably arranged eccentrically. Accordingly, the sleeve-like area 23 c comprises a thicker side that defines an indication projection 21.

FIG. 20 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2 a, which is at least partly embodied with a varying or reduced outer circumference 2 c, e.g. by means of an annular recess. The reduced outer circumference 2 c and the normal outer circumference 2 b of the bolt shaft 2 a as such are preferably arranged concentrically with the normal outer circumference 2 b being elliptical.

On the other hand, FIG. 20 represents the bushing 23 of FIG. 15, which comprises the opening 23 a. The latter is annular and arranged concentrically to the sleeve-like area 23 c of the bushing 23, which is preferably elliptical.

It should be noted that the above described, preferred embodiments are merely described to illustrate possible embodiments of the present invention, but not in order to restrict the present invention thereto. Instead, multiple modifications and variations of the invention are possible and should, therefore, also be considered as being part of the invention. In particular, combinations of the above described variations are possible and should, thus, be considered as being covered by the present invention. In particular, such variations and modifications are readily available to the person skilled in the art and may become necessary dependent e.g. on an underlying number of load directions and other application-specific parameters, such as a predetermined edge distance, number of bolts and/or bushings to be used, and so on.

REFERENCE LIST

-   1 load-introduction joint -   2 bolt -   2 a bolt shaft -   2 b bolt shaft outer circumference -   2 c reduced bolt shaft outer circumference -   3 nut -   4 first attachment component -   4 a first attachment component opening -   5 second attachment component -   5 a second attachment component opening -   6 composite laminate -   6 a composite laminate opening -   6 b composite laminate upper side -   6 c composite laminate lower side -   6 d composite laminate opening center -   6 e composite laminate end edge -   6 f composite laminate longitudinal edge -   6 g peripheral opening side -   7 composite laminate prefailure indication element -   7 a prefailure indication projection -   7 b upper axial recess -   7 c lower axial recess -   7 d prefailure indication projection thickness -   8 first bushing -   8 a first bushing opening -   8 b first bushing outer circumference -   8 c first bushing sleeve-like area -   9 second bushing -   9 a second bushing opening -   9 b second bushing outer circumference -   9 c second bushing sleeve-like area -   10 main load direction -   11 a end edge distance -   11 b longitudinal edge distance -   11 c composite laminate opening diameter -   11 d composite laminate thickness -   11 e prefailure indication projection length -   12 a initial bolt/bushing position -   12 b prefailure indication bolt/bushing position -   13 prefailure gap -   14 a transversal composite laminate rupture -   14 b longitudinal composite laminate rupture -   15 a upper axial chamfer -   15 b lower axial chamfer -   16 undercut -   17 prefailure layer -   18 tangential recesses -   19 peripheral recesses -   20 peripheral cut-outs -   20 a middle axis -   21 bushing indication projection -   23 third bushing -   23 a third bushing opening -   23 b third bushing outer circumference -   23 c third bushing sleeve-like area -   F main load 

What is claimed is:
 1. A composite laminate for connection to an attachment component in a load-introduction joint, the composite laminate comprising: an opening that comprises an inner peripheral sidewall and extends between opposing sides of the composite laminate, the opening being adapted to receive a load-introduction component of the load-introduction joint therethrough such that the load-introduction component comes at least partly in direct contact with the inner peripheral sidewall of the opening in a normal operation mode of the load-introduction joint; and a prefailure indication element that is arranged inside the opening on a portion of the inner peripheral sidewall of the opening, the prefailure indication element being modifiable by the load-introduction component in a prefailure mode of the load-introduction joint from a first condition in the normal operation mode of the load-introduction joint to a second condition in the prefailure mode for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication element is compressed or partially destructed by the load-introduction component of the load-introduction joint in the prefailure mode from the first condition in the normal operation mode to the second condition in the prefailure mode, wherein the prefailure indication element comprises at least one prefailure indication projection that is arranged on the inner peripheral sidewall of the opening, wherein the at least one prefailure indication projection is arc-shaped, and wherein the at least one arc-shaped prefailure indication projection is arranged between a first and a second arc-shaped axial recess provided inside the opening on the inner peripheral sidewall of the opening.
 2. The composite laminate of claim 1, wherein the opening comprises a first opening diameter when the prefailure indication element is in the first condition in the normal operation mode and a second opening diameter when the prefailure indication element is in the second condition in the prefailure mode, and wherein the first opening diameter is no more than 50% of the second opening diameter.
 3. The composite laminate of claim 1, wherein the prefailure indication element is provided inside the opening at least on a peripheral side of the opening that is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.
 4. A load-introduction joint, comprising: an attachment component; a load-introduction component; and a composite laminate that is mounted to the attachment component via the load-introduction component, the composite laminate comprising: an opening that comprises an inner peripheral sidewall and extends between opposing sides of the composite laminate, wherein the opening accommodates at least partly the load-introduction component such that the load-introduction component is at least partly in direct contact with the inner peripheral sidewall of the opening in a normal operation mode of the load-introduction joint; and a prefailure indication element that is arranged inside the opening on a portion of the inner peripheral sidewall of the opening, the prefailure indication element being modifiable by the load-introduction component in a prefailure mode of the load-introduction joint from a first condition in the normal operation mode of the load-introduction joint to a second condition in the prefailure mode for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication element of the composite laminate is compressible or partially destructible by the load-introduction component in the prefailure mode from the first condition in the normal operation mode to the second condition in the prefailure mode and comprises at least one prefailure indication projection that is arranged on the inner peripheral sidewall of the opening of the composite laminate, wherein the at least one prefailure indication projection is arc-shaped, and wherein the at least one arc-shaped prefailure indication projection is arranged between a first and a second arc-shaped axial recess provided inside the opening of the composite laminate on the inner peripheral sidewall of the opening.
 5. The load-introduction joint of claim 4, wherein the opening of the composite laminate comprises a first opening diameter when the prefailure indication element is in the first condition in the normal operation mode and a second opening diameter when the prefailure indication element is in the second condition in the prefailure mode, and wherein the first opening diameter is no more than 50% of the second opening diameter.
 6. The load-introduction joint of claim 4, wherein the prefailure indication element of the composite laminate is provided inside the opening of the composite laminate at least on a peripheral side of the opening that is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.
 7. A composite laminate for connection to at least one attachment component in a load-introduction joint, the composite laminate comprising: an opening extending between opposing sides of the composite laminate and having a central axis thereof, the opening being adapted to receive a load-introduction component of the load-introduction joint; and a prefailure indication projection defined in and extending along a portion of a peripheral sidewall of the opening, the prefailure indication projection having a first position and a second position, the projection being compressed or partially destructed by the load-introduction component when in the second position relative to when in the first position for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication projection comprises a projection defined between axially opposing first and second recesses that extend radially away from the central axis into the peripheral sidewall of the opening, and wherein the prefailure indication projection extends radially toward the central axis between the first and second recesses.
 8. The composite laminate according to claim 7, wherein the prefailure indication projection has a thickness of 50% to 90% of a thickness of the composite laminate adjacent to the load-introduction component when the prefailure indication projection is in the first position.
 9. The composite laminate according to claim 7, wherein the prefailure indication projection is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.
 10. The composite laminate according to claim 7, wherein the prefailure indication projection is formed to reduce a given contact surface between the load-introduction component and the composite laminate when the prefailure indication projection is in the first position.
 11. The composite laminate according to claim 7, wherein the at least one prefailure indication projection is an arc-shaped prefailure indication projection that extends adjacent to the load-introduction component about less than an entire circumference of the load-introduction component when the arc-shaped prefailure indication projection is in the first position.
 12. The composite laminate according to claim 11, wherein the at least one arc-shaped prefailure indication projection extends in a thickness direction parallel to the central axis of the opening between the first and second recesses. 